PROJECT SUMMARY This application is for F31 support for Rachel Underwood for the remainder of her PhD training. Parkinson's disease (PD) is the second most common neurodegenerative disorder that causes tremendous economic and social burden worldwide. Alpha-synuclein (?syn) is the key protein component of intracellular aggregates termed Lewy bodies, the pathological hallmark of Parkinson's disease2. In its non-native form, ?syn is thought to propagate throughout the brain in a prion-like fashion: when released, it can be taken up by surrounding neurons to cause aggregation and cell death by inducing the misfolding of endogenous ?syn3-6. However, the mechanism that regulates the release and spread of ?syn is poorly understood. ?Syn release has previously been shown to involve exosomal and non-exosomal pathways involving endosomes which are tightly regulated by a family of GTPase proteins termed Rabs8-12. Rab27b is a GTPase protein that regulates protein secretion through both exosomal and non-exosomal pathways and is highly expressed in the human brain13-15. In addition, Rab27b polymorphisms have been associated with a higher motor neuron disease risk in GWAS studies16. Rab27b transcriptional dysregulation has also been associated with other neurological disorders including Alzheimer's disease, Dementia with Lewy Bodies and X-linked dystonia parkinsonism syndrome17-19. Our data shows that Rab27b expression is increased in human PD postmortem brain tissue in comparison to age-matched healthy controls (Fig. 4). Our preliminary data also indicates that Rab27b modulates the release of ?syn in vitro (Fig. 5). Our central hypothesis is that Rab27b promotes ?syn- mediated neurotoxicity by inducing the release and spread of toxic ?syn species. The goal of this proposal is to determine the effects of Rab27b on ?syn release, spread, and paracrine toxicity. In Aim 1, we will test if Rab27b regulates ?syn release and toxicity in a paracrine tetracycline-inducible ?syn cell model, in which ?syn, upon induction with doxycycline, is released into the conditioned media (CM) and induces cell death when transferred to separately cultured neurons. We will use knockdown and dominant negative technology to test if Rab27b regulates ?syn release, clearance, and toxicity in this model. In Aim 2, we will test if Rab27b regulates ?syn aggregation, spread, and neurotoxicity in cellular and in vivo ?syn fibril based PD models. If our hypothesis is true, Rab27b may prove to be a useful research tool and a potential target for therapeutic intervention to reduce ?syn propagation in PD.